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Thread: Can anyone figured this out

  1. #1 Can anyone figured this out 
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    A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

    Will the plane fly?


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  3. #2  
    Forum Cosmic Wizard SkinWalker's Avatar
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    No. It is standing still relative to the air molecules that it will need to increased velocity of in order to gain lift under the wings. It isn't about how fast the planes wheels move, its about how fast it wings move relative to the air molecules. More air molecules/sec will increase the lift potential.


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  4. #3  
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    Yes, the conveyer belt will not impede the forward motion of the plane.

    The forward momentum is not supplied by the wheels but by the thrust of the propellers or turbines, therefore there is no way to "tune" the planes forward momentum to that of the to the conveyor belt. There is no attachment.
    The conveyor will move one direction the plane will move in the other and fly.
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  5. #4  
    Universal Mind John Galt's Avatar
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    This is incorrect. Skinwalker has given the correct answer.
    We are not tuning the planes motion to the conveyer, we are, as bt redliner made clear, adjusting the motion of the conveyor to match that of the plane. If the plane moves forward at 30mph, for example, the conveyor moves back at 30mph. Relative to the conveyor the plane is moving at 30mph. Relative to the air it is stationary.
    It will not fly as there will be no air motion over the wings to provide lift.
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    Forum Radioactive Isotope mitchellmckain's Avatar
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    Actually I believe that Metatron is correct. The conveyor may present some resistance to the planes motion but if it only moves backwards at a rate equal to the planes forward motion then it will not be very much. A plane is different than a car during takeoff. A car relies on the friction between the wheels and the road in order to move the plane does not. So as the plane engages its engines for takeoff the only effect of the conveyor is that the wheels will have to spin that much faster.

    The conveyor belt is effectively simulating a slippery surface where the turning of the wheels has no relationship with the forward motion. Think about a car on perfectly slipper ice. The wheels turn but nothing happens. But as long as the plane is positioned and properly lined up on the runway, the runway could be covered in ice and it will not impede takeoff much at all. Rember that we have special planes that can take off from water where there is no traction at all (only resistance), and they do not use propellers in the water like a motorboat.
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  7. #6  
    Universal Mind John Galt's Avatar
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    Mitchell, you are the physicist, so I incline strongly towards accepting your view on this. However, it appears as if the requirement is that the plane not 'engage' with the conveyer, and this is predicated upon the surface being 'slippy'. I do not see that as part of the original problem, as stated. What am I missing?
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  8. #7  
    Forum Radioactive Isotope mitchellmckain's Avatar
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    Quote Originally Posted by Ophiolite
    Mitchell, you are the physicist, so I incline strongly towards accepting your view on this. However, it appears as if the requirement is that the plane not 'engage' with the conveyer, and this is predicated upon the surface being 'slippy'. I do not see that as part of the original problem, as stated. What am I missing?
    Well I can thank your knowlege of my credentials for giving me another chance at least. Physics should not be decided by credentials, but at least I can try explaining once more.

    Consider why ice is slippery. When the tires grab onto the ice and puts pressure on it, it melts and is simply carried backward by the tire. In this sense the ice acts a bit like the conveyor on which we are proposing to put the plane on. It just moves backwards automatically without our interference.

    Imagine being on treadmill that moves in response to the slightest pressure. It would be hard to stand up on such a treadmill. It would be "slippery". In this case there is no need to match the forward motion artifiicially, it would so automatically.

    Or how about this. Imagine being on a platform that sits on the kind of air cushion used in air hockey. Furthermore suppose the platform is really lightweight like styrofoam, so that the total mass of the platform is much less than your mass. There is traction between your feet and the platform but any attempt by you to move will be fruitless because the traction only causes the platform to move while you remain motionless. Does this not sound a lot like what the conveyor would be doing? Again despite the traction between your feet and the platform, it will seem slippery and it will be hard to keep standing. Now suppose you have a pressurized air tank, then by releasing the air from the tank you will move forward easily. Is this not a lot like what the engine of the plane does?

    The conveyor only does this in one direction and we artificially speed up the conveyor to try to conteract the plane engines but there is really very little real difference between these cases. Ok, but what about a person with an air tank on the conveyor? Well the plane would have a problem without wheels, so to be fair, we need to put the person in roller blades before we put him on the conveyor with an air tank. Now when the person opens the air tank he begins to move forward. When we try to make the conveyor move backwards, the only effect will be to make the wheels of the roller blades spin faster, just like I said would happen with the the airplane.

    The only function of wheels of the plane which touch the conveyor is make it so the plane moves forward with minimal friction, a slippery surface works just as well. This is why a plane landing on water or snow doesn't need wheels.

    Ok how about some solid physics? Whether it is the jet engines, propellors or the air tank, the momentum of the air being propelled in one direction must be balanced by an equal and opposite momentum in the plane or in the guy on roller blades. The only way that the conveyor belt could take away this momentum is by exerting a force on the plane or guy in roller blades. But the wheels prevent this by spinning freely with very little friction no matter how fast the conveyor moves backwards.
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  9. #8  
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    Ophiolite wrote;
    Mitchell, you are the physicist, so I incline strongly towards accepting your view on this.
    Engineer’s never get any respect. :?

    Thanks for the ice analogy mitchellmckain it makes it simple, I never thought of that. I saw that the wheels would turn at speed of the conveyor plus the speed of the forward motion of the plane. The conveyor’s motion is only serving to rotate the wheels faster not impede any forward thrust.
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  10. #9  
    Universal Mind John Galt's Avatar
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    This demonstrates that geologists and anthropologists should not be listened to on all topics. On the plus side they lead much more interesting lives. :wink:
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  11. #10 Re: Can anyone figured this out 
    Forum Freshman genep's Avatar
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    Quote Originally Posted by bt redliner
    A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

    Will the plane fly?
    the UNCERTAINTY PRINCIPLE says that there is a chance that it will fly but it is far more likely to vanish before it does.
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